add more SDK checks: build-tools/19.0.3 and presense of aapt

Make sure that fdroid can find aapt in the current config, otherwise exit
with an error.  Some users don't have build_tools set, and their SDK does
not include the build-tools in the default versioned dir, so this should
warn them of what is wrong.

fdroidserver/common.py

@@ -144,6 +144,23 @@ def test_sdk_exists(c):
     return True
 
 
+def test_build_tools_exists(c):
+    if not test_sdk_exists(c):
+        return False
+    build_tools = os.path.join(c['sdk_path'], 'build-tools')
+    versioned_build_tools = os.path.join(build_tools, c['build_tools'])
+    if not os.path.isdir(versioned_build_tools):
+        logging.critical('Android Build Tools path "'
+                         + versioned_build_tools + '" does not exist!')
+        return False
+    if not os.path.exists(os.path.join(c['sdk_path'], 'build-tools', c['build_tools'], 'aapt')):
+        logging.critical('Android Build Tools "'
+                         + versioned_build_tools
+                         + '" does not contain "aapt"!')
+        return False
+    return True
+
+
 def write_password_file(pwtype, password=None):
     '''
     writes out passwords to a protected file instead of passing passwords as